Fluorescent metabolic markers which accumulate in certain pathologically altered regions of the body are known, for example, in the case of tumors, inflammations or other foci of infection. In addition, fluorescent metabolic markers are known which can be distributed everywhere in the body but which are activated only in certain body regions, by for example, tumor-specific enzyme activities. For this purpose, the metabolic marker is administered to the patient in the form of a fluorescent dye and this fluorescent dye is administered into the bloodstream, for example.